# Check a shelf for sag

Tell KerfCraft what a shelf holds, read how much it sags now and in time, and choose a fix.

KerfCraft estimates how much a shelf bends under its load, now and after years. The check is advice from simple beam sums and estimated loads. It does not tell you that a shelf is safe.

## Set the load on a shelf

1. Select a shelf.
2. In the **Properties** panel, under **Manufacturing**, find **Shelf load**.
3. Select what the shelf holds, for example **Paperbacks** or **Vinyl records**.
4. If no preset is correct, select **custom kg/m** and type the load in kg per metre.

Each preset shows its estimated load in kg/m. In a project in feet and inches, the loads are in lb/ft.

## Read the result

1. Open the **Cut list** tab of the manufacturing panel.
2. Find the line for the shelf below the cut list.
3. Click **Fixes and assumptions** to read the details.

If the shelf is fine, there are no fixes, so click **Assumptions** instead. A line starts with ✓ when the shelf is fine, and with ⚠ when it is not. It gives the sag now and in time, as a fraction of the span. The fixes are in order, from the smallest change:

- a thicker board,
- a centre support,
- a stiffer material of the same thickness,
- a hardwood lip on the front edge. KerfCraft does not calculate this one, so check it yourself.

If KerfCraft has no stiffness and strength data for the material, the line says "Can't check this material".

## Try a shelf here

This calculator uses the same check as the app, for a sheet-material shelf. It takes the thickness from the material.

## What the check assumes

- The shelf is a plain board, supported at both ends. Fixed ends sag less, so the result is on the safe side.
- The load sits on the shelf for years. Long-term sag comes from Eurocode 5 creep factors for heated rooms indoors.
- "Fine" is a long-term sag of at most 1/600 of the span. "Visible sag" is up to 1/300. More than that "will bow badly".
- A separate strength check gives a warning when the bending stress is too high.
- Front lipping and back cleats are not included. They make the shelf stiffer.

For a generated shelf, the span is the length of the shelf. The real span between the pins is a little shorter.

## Tall units

A freestanding carcass or PAX-size frame more than twice as tall as it is deep shows "Fix this to the wall so it can't tip". KerfCraft does not calculate stability.

## Related

- [Build a cabinet, bookcase or wardrobe](/docs/how-to/furniture/build-a-carcass/)
- [Choose materials](/docs/how-to/furniture/choose-materials/)